The objective of this project is to elucidate the molecular nature of site-specific instability in the X chromosome of Drosophila melanogaster. The site of interest is the 6F1-2 doublet and a few chromomeres flanking the doublet. Our first aim is to isolate a DNA sequence in or very near the 6F1-2 doublet. The second aim is to clone the DNA sequences corresponding to the 6F1-2 doublet in the X chromosome of 59b-z and its hypermutable derivatives, 59b-z Uc, 59b-z Uc-1, and 59b-z Uc-1r. The molecular dissection of the clones by restriction mapping, EM visualization, blotting, base sequencing, and in situ hybridization will be our third aim. The site-specific instability in these chromosomes is hereditary, and is associated with chromosome rearrangements that indicate the involvement of a dtransposable destabilizing element in the 6F1-2 doublet. It is our hope that by unravelling the molecular nature of the breakage sites and relating them to the existing cytogenetic data, we will obtain a fresh perspective for understanding mutagenesis, oncogenesis, and evolution. One of our long-term objectives is to use the clones for study of site-specific mutations and chromosome breakage events, and assessing their impact on the Drosophila genome. The results of such studies using different classes of chromosomes, including the chromosomes that are constantly generating chromosome rearrangements (such as our unstable chromosomes), may shed some new insight into the structural aspects and properties of chromosomes and genes which are not familiar to us at present.